LOANS AND ALLOWANCE FOR CREDIT LOSSES - Additional Information (Details) - USD ($)		3 Months Ended		12 Months Ended
	Jan. 01, 2021	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020
Allowance for credit losses	$ 67,300,000
Troubled Debt Restructurings Internal Grading System		$ 3,300,000		$ 1,600,000
Financing Receivables, Troubled Debt Restructuring		4,000,000		3,300,000
Impaired Loans For Specific Valuation Allowances				713,000
Impaired Loans				4,800,000
Non-accrual loans for no related allowance amortized cost		3,500,000
TDR loans evaluated on individual	100,000
After-tax effect decreased retained earnings	14,200,000
Troubled Debt Restructurings Returned to Accrual Status		27,000	$ 0
Loans and Leases Receivable, Allowance		67,702,000		55,743,000
Provision for Loan and Lease Losses		300,000	3,871,000
Aggregate Principal balance outstanding 19 modified comercial loans		141,000,000
Aggregate Principal balance outstanding 92 modified comercial loans		5,000,000
Aggregate Principal balance outstanding 65 modified comercial loans				233,000,000
Aggregate Principal balance outstanding 581 modified comercial loans				18,000,000
FDIC-assisted acquired loans class categories		91,900,000
FDIC-acquired loans and remaining accretable yield		1,300,000
FDIC-assisted acquired loans not loan class categories		98,600,000
Minimum loan balance with addition to TDR status for classification of loans into Collateral-dependent loans		100,000
Hotel [Member]
Total modified loans by collateral		69,000,000
Multifamily [Member]
Total modified loans by collateral		11,000,000
Retail Site [Member]
Total modified loans by collateral		22,000,000
Health Care [Member]
Total modified loans by collateral		28,000,000
Allowance for unfunded commitment
Allowance for credit losses	8,700,000
PCI [Member]
Allowance for credit losses	1,900,000
Adopted ASC 326 [Member]
Loans and Leases Receivable, Allowance	11,600,000
Loans acquired and accounted		91,900,000
Adopted ASC 310-30 [Member]
Non-accruing loans acquired		3,600,000		3,800,000
FDIC-acquired Loans Accounted				$ 1,700,000
CECL Methodology [Member]
Allowance for credit losses	11,600,000		$ 3,900,000
Allowance for credit loss remaining discount on loan	1,900,000
Provision for Loan and Lease Losses		300,000
CECL Methodology [Member] | Allowance for unfunded commitment
Allowance for credit losses	$ 8,700,000
Benefit for unfunded loan commitments		$ 674,000